1. Field of the Invention
The present invention relates to a DC-DC converter. Specifically, the present invention relates to a DC-DC converter capable of keeping input impedance constant. Further, the present invention relates to a power receiving device including the DC-DC converter. Furthermore, the present invention relates to a power feeding system including the power receiving device.
2. Description of the Related Art
A method called a magnetic resonance method is attracting attention as a method for feeding power to an object (hereinafter, also referred to as a power receiving device) in a state where contact with a power supply source (hereinafter, also referred to as a power transmitting device) is not made (such a method is also referred to as contactless power feeding, wireless feeding, or the like). The magnetic resonance method is a method for forming an energy propagation path by providing resonator coupling between resonance coils each of which is provided in a power transmitting device and a power receiving device. The magnetic resonance method has a longer power transmittable distance than other methods capable of contactless power feeding (e.g., an electromagnetic induction method and an electrostatic induction method). For example, Non-Patent Document 1 discloses that in the magnetic resonance method, transmission efficiency is approximately 90% when the distance between a pair of resonance coils is 1 m and that the transmission efficiency is approximately 45% when the distance between the pair of resonance coils is 2 m.